Process for chemical plating



Jan. 28, 1969 HElNZ-GUNTER KLEIN ETAL 3,424,660

PROCESS FOR CHEMICAL PLATING Fil'ed Jan. 8, 1965 Sheet 1 r 5 A/Cm L Tr 0-3.

-z0 200 +4220 +6 00 *abo 1600 mV INVENTORSI HE/NZ GUNTHER KLEIN, KONRAD LANG, ED/TH-LU/SE SCHMEL/NG, HELMUT WEISSBACH.

W WW

ATTOPNEY' Jan. 28, 1969 HElNZ-GUNTER K LEIN ETAL 3, 0

PROCESS FOR CHEMICAL PLATING Filed Jan. 8, 1965 Sheet g of 5 5 +200 +000 0' b +1000 mV 10' A/cm 10"; q ID INVENTORS HE/NZ GUNTHEP KLEIN, KONRAD LANG, EDITH LU/SE SCHMEL/NG, HELMUT WEISSBACH.

AITORNE Y J n- 8, 1969 HEINZGUNTER KLEIN ETAL 3,424,660

PROCESS FOR CHEMICAL PLATING Filed Jan. 8, 1965 Sheet 3 of5 l n I l l l FlG 6 INVENTORS HE/NZ GUNTHEP KL E/N, KONPAD LANG, EDITH LU/SE SCHMELING, HELMUT WEISSBACH.

BY %M "M ATTORNEY United States Patent fice 3,424,660 Patented Jan. 28, 1969 F 41,743 US. Cl. 204-447 22 Claims Int. (:1. C23b 5/18 ABSTRACT OF THE DISCLOSURE Process for electroless metal plating of metallic and non-metallic articles with plating baths of chemical reducing agents in which the article is contacted with the chemical reducing agent plating bath such that metal is deposited from the bath onto the article to plate such article, in which particular metallic surfaces in contact with the chemical reducing agent bath are protected against chemical plating by such bath, wherein:

(a) The metallic surfaces to be protected are connected in an electric circuit which contains additionally the plating bath as electrolyte, which bath contains a chemical reducing agent such as alkali metal borohydride, sodium hypophosphite, a borazane or a borazole, as well as at least one counterelectrode and at least one voltage source of direct current, such that (b) An electric potential isapplied to the metallic surfaces to be protected as the anode and to the counterelectrode as the cathode, which potential corresponds to the protection potential region lying between the mottling potential and the transpassivity on the current density/ potential curve (or which corresponds to the rest potential on such curve), and

(c) Adjusting the current density to a value of not more than about 1(l a./crn.

The present invention relates to an improved process for chemical plating, and more particularly to an improvement in a chemical plating process by which specific metal surfaces, which are in contact with the chemical plating bath but are not intended to be plated, may be protected against plating while other metal or metallic or non-metal or non-metallic surfaces which are intended to be plated may be plated by such bath.

Metals such as nickel, cobalt and/or iron can be deposited on catalytically active surfaces by reducing agents such as sodium hypophosphite, sodium borohydride, borazanes or other boron hydride compounds. This so called chemical plating is performed very easily on metal surfaces but non'metallic surfaces such as glass, ceramic, graphite or synthetic resins can also be coated with metal by chemical processes after activation by easily reducible metal salts. Surfaces without any catalytic activity are very often so activated by prolonged exposure to the action of plating baths. It is in this connection that the present process is of significance as the same more precisely relates to an improved process for the protection of particular metal surfaces which are in contact with chemical plating baths.

The choice of a suitable constructional material for the production of plating apparatus is connected with considerable problems. Tanks made of glass, a material which is in itself practically inactive, can only be manufactured up to a certain, relativelysmall size; and moreover, glass tanks have the known disadvantages, as they are too sensi tive to mechanical and temperature stresses. Graphite, synthetic resins and rubberized surfaces tend to undergo autocatalytic plating.

The construction of plating apparatus of stainless Cr-Ni steels rendered chemically passive, achieves only a soon terminated success, as the passive layer before long is destroyed by the strongly reducing bath, whereupon deposition of metal on the walls of the container occurs after a comparatively short time. The apparatus must then be treated with nitric acid or mixtures of nitric acid and hydrofluoric acid to remove the deposited metal and to render the surface of the apparatus passive again, as described in the literature. The process is very complicated and uneconomic due to servicing and prolonged times of standstill.

No material has hitherto become known which remains absolutely resistant to chemical plating.

It is therefore an object of the present invention to improve the process for chemical plating known per se by protecting particular metal surfaces or metal coated surfaces against chemical plating.

It has been found in accordance with the present invention that such protection can be achieved if these metallic surfaces are connected into an electric circuit which contains one or more counter electrodes, the liquid of the plating bath as electrolyte and a voltage source of direct current, and wherein the metallic surfaces are polarized in known manner, the potential being adjusted to a value which corresponds to the rest potential or the protection potential range on the current density/ potential curve, said potential range being in the range of between the mottling potential and the transpassivity, the current intensity assuming a value of not more than 10* a./cm. It has been found that no deposition of metal occurs on metal surfaces in contact with plating solutions if the surface is polarized as outlined above. In practice, the current intensity is changed with the aid of a control instrument and one or more standard electrodes immersed in the plating bath until the desired polarization potential has become established, or the potential or a potential range is varied until the current intensity has reached a minimum value.

It is surprising and especially advantageous for practical use that, in contrast to the passivation produced by chemical means, the electrolytically produced polarization imparts a permanent protection against chemical plating in spite of the fact that one would expect the strong reducing agents contained in the plating bath to have a depolarizing effect. Furthermore it has been found that by a suitable selection of material the current density is very low so that no marked corrosion of the parts of the apparatus such as tanks for the bath, pumps, heat exchangers, storage and buffer vessels, purification apparatus etc. occurs. This latter effect increases the economy of the process considerably.

In the instant drawings, FIGS. 1 to 4 illustrate graphically' current density/potential curves for plating baths contemplated in accordance with the present invention, and

FIGS. 5 and 6 illustrate electrode arrangements usable in plating bath environments contemplated by the present invention.

To determine the most suitable polarization potential for a particular combination of material to be protected and plating bath, a current density/ potential curve is plotted in known manner by the potentiostatic, potentiokinetic or galvanostatic method, as indicated schematically in FIGURE 1. The current density increases in a characteristic manner with increasing potential. In the cathodic region, it first follows Tafels straight line (T in FIGURE 1) in which hydrogen and metal may be deposited. At GR it passes through the so called rest potential where the external polarization is zero. As the negativity of the potential is decreased, the anodic part of the curve begins at ER, as does the anodic polarization region in its wider sense. From 6R onwards some protection against chemical plating exists but since a sometimes quite considerable current density associated with a correspondingly high removal i.e. chemical dissolution, of material occurs between the rest potential and the Flade potential or mottling potential 61:, the region of the curve from 63 to about the mottling potential is less suitable for practical use. If it is possible to maintain the rest potential ER very accurately, this potential can be used for providing protection against chemical nickel plating. After the current density has passed through the mottling potential in the direction towards a more posi tive potential, it falls rapidly, In the subsequent part S, referred to hereinafter as the protective potential region curve in the range between the mottling potential and the transpassivity, the current density I referred to hereinafter as the protective current density, which is in many cases very low, is almost independent of the potential. Protection against chemical plating can most easily and reliably be carried out in the above defined protective potential region S since the region S is usually very wide and fluctuation in potential do not have any adverse efiects. Hence, working in this region is very reliable. The increase in current as the potential increases still further after the region S, an increase defined as transpassivity Tr, indicates the dissolution of the metal in its highest state of oxidation. If relatively large quantities of chloride ions are contained in the electrolyte an increase in current frequently occurs before the potential corresponding to transpassivity is reached. This increase is also known as branched pitting corrosion L which indicates the formation of pitted corrosion.

For practical reasons, the work is carried out at a potential or potential region lying in the range between the mottling potential and the transpassivity.

The position of the range of protective potential S and the magnitude of the polarization current density I is characteristic for individual materials and also depends on the type, concentration and temperature of the plating baths used as electrolytes. It can be determined by the generally known method for determining the current density/potential curve.

To satisfy technical requirements, the protective current density I should be below about a./cm. so that the anodic dissolution of the metal can be kept within tolerable limits. In the case of iron, this corresponds to a removal of metal of 28.5 g./(m. .day). According to Todts Korrosion and Korrosionsschutz, page 1227, a heavy metal which undergoes dissolution at the rate of about 21 g./(m. .day) can still be regarded as sufficiently stable.

Th current density/potential curve for 18/8 Cr-Ni steel in a plating bath of composition 30 g./l. of nickel chloride, 40 g./l. of sodium hydroxide, 60 g./l. of ethylene diamine of concentration 98%, 3 g./l. of sodium fluoride and 0.6 g./l, of sodium borohydride at 90 C. is shown in FIGURE 2. The current density/potential curve for 18/8 Cr-Ni steel in a bath of the composition 30 g./l. of nickel chloride, 10 g./l. of sodium citrate, 10 g./l. of trimethyl borazane at 65 C. and pH 5.3 is shown in FIGURE 3. The course of the current density/ potential curve for 18/ 10/2 =Cr-Ni-Mo steel in a plating bath based on sodium hypophosphite and containing 30 g./l. of nickel chloride, 10 g./l. of sodium citrate, 10 g./l. of sodium hypophosphite at 90 C. and pH 4 to 5 is shown in FIGURE 4. In FIGURE 2-4 the abscissae denote the voltages in (mv.) millivolt referred to thalamide electrodes and the ordinates denote the current density in a./cm.

Table I shows the characteristic values of the rest potential ER, protective potential region S, maximum current density at mottling potential, i.e., Flade potential 5 and protective current density I obtained for various chemical plating baths from the potentiokinetically determined (change in potential: 200 mv./ h.) current density/ 4 potential curves for 18/ 8 Cr-Ni steel, 18/ 10/2 Cr-Ni-Mo steel, 18/18/2/2 Cr-Ni-Mo'Cu steel, an 18% Cr steel and a nickel-boron alloy (about 94% Ni, 6% B); deposited in a plating bath based on sodium borohydride of the composition shown in Table I.

The metals and alloys which can be subjected to the process according to the invention are by no means limited to the materials shown in Table I and mentioned in the examples; in fact, any anodically polarizable metals and metal alloys can be used whose protective current density I lies in the region of about 10- a./cm. or below. Whereas the limitation described above applies to the choice of materials used for the containers etc., the composition, concentration and temperature of the plating baths used are not critical for carrying out the process of the invention. For example, any of the plating baths known in the literature, using sodium hypophosphite, alkali metal borohydrides, boron hydrides and/ or boron hydride compounds such as borazanes and borazoles as reducing agents, may be used. Since protection against plating is imparted over a relatively wide range of potentials, the fluctuations in concentration occurring during the plating process, especially in intermittent operations, are of no importance.

A preferred method of carrying out the invention will be explained with the aid of the following example:

A counter electrode of pure nickel is installed centrally in a cylindrical vessel of 18/ 8 Cr-Ni steel having an (inner) surface of about 0.1 m wetted with electrolyte (plating liquid) and a standard electrode of the same material as the counter electrode is installed near the wall of the vessel. A Wenking potentiostat is used as source of direct current voltage and as control instrument. Owing to the good conductivity (about 0.3 ohmcm? at C.) of the plating bath used as electrolyte, which has a composition of 30 g./l. of nickel chloride, 40 g./l. of sodium hydroxide, 6O g./l. of ethylene diamine, 30 g./l. of sodium fluoride and 0.6 g./l. of sodium borohydride in aqueous solution, the distance of the electrodes from the wall of the vessel is of no significance in a plating vessel of such small dimensions. A drop in potential would be noticed only at distances of about 1 m. by means of the potentiostat, a potential of +600 mv. against the nickel standard electrode is imparted to the wall of the vessel, this potential having been determined previously by current density/ potential measurements. The current density which becomes established in the process is about 50 ma./ m. Plating was carried out continuously for 3 days in this vessel. No deposition of nickel occurred on the walls of the vessel. No loss in weight of the material of the vessel could be determined. No pitting corrosion occurred in spite of the relatively high chloride concentration.

One advantage for the technical application of the process of the invention is that the system can be adjusted to the desired range of passivity with relatively weak electric currents, on an average about 0.05 a./m.

As experiments have shown, standard electrodes of the usual form such as mercury-calomel electrodes, are less suitable for use in chemical plating baths because they become inactive due to deposition of metal. It was found, surprisingly that it is also possible to use metals or alloys if desired of the same type as the metal to be plated as standard electrodes in the process of invention.

For example, in a nickel plating bath, standard electrodes are used which are made of nickel or of a nickelboron or nickel-phosphorus alloy which corresponds to the composition of the nickel alloy chemically deposited from the bath. It is immaterial whether the standard electrode is made from a molten nickel alloy or from a nickel alloy applied by chemical plating on a metal or nonmetallic material. Thus, for example, it is possible to use standard electrodes made of copper, brass, iron, silicon carbide, graphite, porcelain, glass or synthetic resins such as polystyrene, which materials have been plated with a chemically non-porous layer of nickel about 50 mg thick.

These standard electrodes maintain the potential imparted to them at a practically constant value and moreover, in contrast to the conventional standard electrodes, they are completely insensitive to deposition of metal and mechanical completely insensitive to deposition of metal and mechanical stresses.

For the counter-electrodes it is preferable to use metals or alloys which correspond substantially to the metal etc. to be deposited. Thus, for example, in a nickel plating bath it is suit-able to use counter-electrodes of pure nickel.

The above mentioned electrodes may, surprisingly, be used not only in plating baths which have a constant metal content but also in intermittently operated chemical plating baths, i.e. in baths in which wide fluctuations in the concentration of metal salts occurs. Thus when using a nickel boride electrode, the potential varies by only about 20 mv. in transition from a freshly prepared plating bath (30 g./l. nickel chloride) to a bath which has been exhausted to a considerable extent by plating (7.2 g./l. nickel chloride). This fluctuation has no adverse effect on the process in practice in view of the great width of the protective potential region S.

Polarization shows no harmful effect on the reducing agents used in chemical plating. In plating processes carried out for the sake of comparison in polarized V2A vessels, no greater consumption of reducing agents was found to take place than in the case of plating carried out in glass containers.

The process of the invention provides a technical advance in that easily accessible and inexpensive metals and alloys can be used for the manufacture of all the containers, pumps, filters, heat exchangers, conductors, valves etc. used for the chemical plating, and the repeated interruption of the operation for removal of the metal deposits and renewed passivation of the material of the container is dispensed with. Losses due to deposition of metal on parts of the plating apparatus no longer occur, with the result that the plating yield and the reliability of the process are considerably increased. Further it is possible for the first time, without the use of covering lacquers, to plate parts of metal surfaces on the one hand and to protect other parts of the metal surfaces against plating on the other hand, provided that these parts of metal surfaces are electrically insulated from each other.

The electrolytic polarization is not only more effective but can also be carried out more elegantly and with less risk of accident than chemical passivation with strong acids and it does not require a specially trained staff.

Example 1 In a cylindrical vessel of 18/8 Cr-Ni steel having a diameter of 15 cm., a height of 20 cm. and an inner surface of about 0.1 m? wetted with electrolyte, a circular road of pure nickel (diameter mm., length 30 cm.) was arranged concentrically as the counter-electrode, and a standard electrode of the same material and some dimensions was arranged in an insulated manner near the wall of the vessel. The counter-electrode was connected as the cathode, the wall of the vessel as the anode to the corresponding current outputs, and the standard electrode was connected to the socket a of a Wenking potentiostat (FIGURES 5 and 6). By means of the potentiostat, a potential of +600 mv. compared with the standard electrode was imparted to the vessel, this potential being derived from the current density/potential curve. 3 liters of a chemical plating solution of the composition 30 g./l. of nickel chloride, 40 g./1. of sodium hydroxide, 60 g./l. of ethylene diamine, 3 g./l. of sodium fluoride, 0.6 g./l. of sodium borohydride and 0.01 g./l. of lead acetate was heated to 90 C. in the vessel. The electrolyte has a specific conductivity of about 0.3 ohm .cm" Two iron plates each having a surface of 10 cm. 10 cm. were then suspended in the bath. During the plating 50 ml./h. of a sodium borohydride solution of 21 g./l. of NaBH in 90 g./l. of NaOH and 50 ml./h. of a metal salt replacement solution of 171 g./l. of nickel chloride, 100 g./l. of ethylene diamine and 5 g./l. of sodium fluoride and 25 ml./h. of a lead acetate solution containing 2 g./l. were run in continuously. Bath solution was removed continuously at the rate of 50 mL/h. The evaporation losses were about ml./h. The rate of deposition was approximately 10-12 ,u/h. The surfaces of the plates covered with Ni-B had a silvery shiny appearance. The current required for protecting the plating vessel was in the region of 2 marl-1 ma. The vessel used for the experiment was completely unaltered after an experimental time of 10 hours and in particular it showed no covering with nickel.

Example 2 In a cylindrical container of 18/8 Cr-Ni steel of the dimensions given in Example 1, having a surface of 0.1 m? wetted with plating bath, a counter-electrode of pure nickel was placed concentrically and a standard electrode of Ni-B alloy was placed near the wall of the vessel. The standard electrode consisted of an earthenware pipe closed at the bottom (diameter 12 mm., length 250 mm.) on which a non-porous coating, 50,41. in thickness, of Ni-B alloy had been deposited by chemical plating. The two electrodes were connected to the potentiostat in the manner described in Example 1. A potential of+ 600 mv. compared with the standard electrode was imparted to the vessel.

3 liters of the plating bath mentioned in Example 1 were placed in the vessel and heated to C. By plating the iron plates suspended in the bath, the nickel was removed from the bath down to a residual content of 6.8 g./l. of nickel chloride.

During the time of operation, the polarization current had an average value of 5 to 8 ma. and did not alter significantly during the time of the experiment in spite of the loss in concentration of the Ni salt. Nickel plating of the Wall of the container did not take place.

Example 3 A pipe of 18/ 8 Cr-Ni steel, 2 m. in length, was provided with 5 inlet connecting tubes, as shown schematically in FIGURE 5, in which the three standard electrodes 1, 2 and 3 and two counter-electrodes 4 and 5, all made of nickel, were inserted in an insulated fashion and connected with potentiostat A. Plating solution was constantly pumped through the pipe by means of a glass circulating pump from a 10 1. glass beaker which contained a plating bath solution of the composition given in Example 1 heated to 90 C. in which sheet irons were continuously nickel plated.

The inner surface of the pipe was about 0.25 m? and it was polarized potentiostatically to +800 mv. The current intensity became established at about 3 ma. After an operating time of 5 days no nickel was deposited on the surface of the pipe.

Example 4 Instead of the cylindrical vessel described in Example 1, a plating bath vat of dimensions 2 m. 1 m. 0.4 m. was used. As shown schematically in FIGURE 6, two counterelectrodes 1 and 2 and two standard electrodes 3 and 4, all made of nickel, were arranged therein, in an insulated manner. The polarization potential compared with the standard electrode was +600 m'v. The current intensity had an average value of 200 ma. The bath was used continuously over a period of 2 weeks without deposition of nickel occurring on the wall of the container.

Example 5 The cylindrcal container of 18/ 8 Cr-Ni steel described in Example 1 was filled with a plating bath solution of the composition 30 g./l. of nickel chloride, 10 g./l. of sodium citrate and 10 g./l. of N-trimethylborazane (pH 5 to 6). A potential of +750 mv. measured against a thallamide electrode, was imparted to the vessel, and the plating bath was then heated to the plating temperature of 65 C. A current of 3 ma. was established. Plating was carried out in known manner with hostaphane foils (i.e., cellulose acetate or cellulose butyrate foils) superficially activated with SnCl /PdCl In this case again there was no deposition of nickel on the walls of the plating vessel.

Example 6 3 liters of plating bath of composition 30 g./l. of nickel chloride, 10 g./l. of sodium citrate and 10 g./l. of sodium hypophosphite (-made up to the full volume with water) were placed in the cylindrical container of 18/8 Cr-Ni steel described in Example 1. The Wall of the container was polarized to a potential of +412 mv. against the nickel standard electrode by means of a potentiostat as described in Example 1. The plating bath was heated to the plating temperature of 93 C., a current of 8 to 9 ma. becoming established.

Two iron plates each having a surface of 10 cm. 10 cm. were plated over a period of hours. During the plating process the pH and the concentration of nickel salt and of sodium hypop'hosphite were kept constant. The plates suspended in the bath were plated uniformly. The walls of the container of noble steel showed no change after plating.

Example 7 Using the apparatus described in Example 1, iron sheets were nickel plated chemically in a bath of composition 45 g./l. of nickel chloride, 100 g./l. of sodium citrate, 50 g./l. of ammonium chloride, 11 g./l. of sodium hypophosphite (pH 8 to at a temperature of 93 C. in the manner described in Example 6. When the wall of the vessel was put at a potential of +420 mv. over the nickel standard electrode, a current of 1 ma. was established. The wall of the plating vessel was not nickel plated.

one counter-electrode and at least one voltage source of direct current,

(b) applying to said metallic surfaces as the anode and said counter-electrode as the cathode an electric potential which corresponds to the protection potential region, said region lying between the mottling potential and the transpassivity on the current density/ potential curve, and

(c) adjusting the current density to a value of not more than about l0 a./cm.

2. Improvement according to claim 1, wherein the potential is regulated potentiostatically by at least one standard electrode immersed in the plating bath.

3. Improvement according to claim 2, wherein the standard electrode is made of a material selected from the group consisting of nickel, cobalt, iron, copper, boron containing and phosphorus-containing alloys thereof, and non-conductive materials coated with a layer of one of said metals and alloys.

4. Improvement according to claim 1, wherein the counter-electrode is made of a material selected from the group consisting of nickel, cobalt, iron, copper, boron containing and phosphorus-containing alloys thereof, and non-conductive materials coated with a layer of one of said metals and alloys.

5. Improvement according to claim 1, wherein the chemical reducing agent is sodium borohydride.

6. Improvement according to claim 1, wherein the chemical reducing agent is sodium hypophosphite.

7. Improvement according to claim 1, wherein the chemical reducing agent is a borazane.

8. Improvement according to claim 1, wherein the chemical reducing agent is a borazole.

9. Improvement according to claim 1, wherein the metals deposited by the plating are selected from the TABLE I.-CURRENT DENSITY/POTENTIAL CURVE INTERPRETATIONS OF VARIOUS MATERIALS IN VARIOUS CHEMICAL PLATING BATHS Rest potential Protective poin mv. against Protective tential region S rent density at current density Maximum cur- Eleetrolyte Material thallamide in mv. against mottling poten- I in a./cm.=

thallamide tial en in aJcm.

3 g./l. nickel chloride 18/8 Cr-Ni-steel +450 +750+900 5, 10-4 1 s 10 g. /l. sodium eitrate 18/10/2 Gr-Ni-Mo steel +520 +550+1, 000 3.10- 10- 10 g./l. sodium hypophosphite 18/18/2/2 CI-Nl-MO II Steel +210 +300+400 2.10- -8. NH t=90 0. pH 4-5 Niglgel-boron alloy (94% Ni, 6% +240 +280--|-450 3. 10-4 +1 -4 1 -4 100 g /1 (1mm I: 18/8 OrNi steel to +1so-+a2o 5 10- +21 (H -10 g. /l. ammonium chloride I 11 gJL Sodium hypophosphitefl 18/10/2 Cr N1 M0 steel +80 +200+300 2.10 3 +81. 0- +6.10- t=90 0. pH 8-10 g0 gfi. nigkel chloride 0 g. so ium ci 10 g Ntrietmlbomane 18/8 O N1 tee +460 +680-+880 8.10 1o t=65 0. pH 5.3

30 nickel chloride: 18/8 Cr-Ni steel i0 +130+600 1 4 1 (1mm 18/10/2 Cr-Ni-Mo steel +150 +500 3, 10-: -2I1 -r g g? lg 18/18/2/2 Or-Ni-Mo-Ou steel :l:0 +120-+700 3. 10- -4.1(r mm e Ni] 3:l)(el-boron alloy (94% Ni, 6% -330 +210--110 5.10- 5.10- 506mm 18 or' steel -250 +230 +400 310-: z. 1 -1 We claim:

1. In a process for electroless metal plating of metallic and non-metallic articles with plating baths of chemical reducing agents in which the article is contacted with the chemical reducing agent plating bath such that metal is deposited from the bath onto the article to plate such article, the improvement which comprises protecting particular metallic surfaces in contact with such chemical reducing agent bath against chemical plating thereby by (a) connecting said metallic surfaces into an electric circuit, which circuit contains additionally the plating bath as electrolyte, said bath containing as chemical reducing agent a member selected from the group consisting of an alkali metal borohydride, sodium group consisting of nickel, cobalt, iron, copper, and boron containing and phosphorus-containing alloys thereof.

10. Improvement according to claim 1, wherein said metallic surfaces to be protected against chemical plating contain the plating bath.

11. Improvement according to claim 10, wherein the metallic surfaces are made of metals having a protective current density of a value of not more than about 10 a./cm.

12. In a process for electroless metal plating of metallic and non-metallic articles with plating baths of chemical reducing agents in which the article is contacted with the chemical reducing agent plating bath such that metal is deposited from the bath onto the article to plate such hypophosphite, a borazane and a borazole, at least article, the improvement which comprises protecting particular metallic surfaces in contact with such chemical reducing agent bath against chemical plating thereby by (a) connecting said metallic surfaces into an electric circuit, which circuit contains additionally the plating bath as electrolyte, said bath containing as chemical reducing agent a member selected from the group consisting of an alkali metal borohydride, sodium hypophosphite, a borazane and a borazole, at least one counter-electrode and at least one voltage source of direct current,

(b) applying to said metallic surfaces as the anode and said counter-electrode as the cathode an electric po tential which corresponds to the rest potential on the current density/ potential curve and (c) adjusting the current density to a value of not more than about 10 a./cm.

13. Improvement according to claim 12, wherein the potential is regulated potentiostatically by at least one standard electrode immersed in the plating bath.

14. Improvement according to claim 13, wherein the standard electrode is made of a material selected from the group consisting of nickel, cobalt, iron, copper, boron containing and phosphorus-containing alloys thereof, and non-conductive materials coated with a layer of one of said metals and alloys.

15. Improvement according to claim 12, wherein the counter-electrode is made of a material selected from the group consisting of nickel, cobalt, iron, copper, boron containing and phosphorus-containing alloys thereof, and non-conductive materials coated with a layer of one of said metals and alloys.

16. Improvement according to claim 12, wherein the chemical reducing agent is sodium borohydride.

17. Improvement according to claim 12, wherein the chemical reducing agent is sodium hypophosphite.

18. Improvement according to claim 12, wherein the chemical reducing agent is a borazane.

19. Improvement according to claim 12, wherein the chemical reducing agent is a borazole.

20. Improvement according to claim 12, wherein the metals deposited by the plating are selected from the group consisting of nickel, cobalt, iron, copper, boron containing and phosphorus-containing alloys thereof.

21. Improvement according to claim 12, wherein said metallic surfaces to be protected against chemical plating contain the plating bath.

22. Improvement according to claim 21, wherein the metallic surfaces are made of metals having a protective current density of a value of not more than about 10 a./cm.

References Cited UNITED STATES PATENTS 2,642,368 6/1953 Wallace et al. 204-147 3,208,925 9/1965 Hutchison et al 204-196 3,216,916 11/1965 Locke 204-196 3,282,819 11/1966 Hovanic 204-213 3,303,111 2/1967 Peach 204-49 3,345,278 10/1967 Mekjean 204-147 3,348,969 10/1967 Katz 117-130 3,350,287 10/1967 Bedi 204-147 JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Exmniner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE "CERTIFICATE OF CORRECTION Patent No. 3,424,660 January 28, 1969 Heinz-Gunter Klein et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4 line 42, "by means" should read By means Columns 7 and 8, TABLE 1, first column, line 1 thereof, "3 g./l. should read 30 g./l. 7 same table sixth column, line 5 thereof, "+2l.O 10 should read +2.lO" lO same table, sixth column, line 6 thereof,

Signed and sealed this 31st day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents 

